1. Field of the Invention
This invention relates generally to concrete structures but more particularly to a water sealing element for concrete joints.
2. Background
Preventing the passage of water through concrete joints is essential for liquid-containing or liquid-excluding structures such as foundation walls, tunnels, swimming pools, reservoirs, water and sewage treatment plants, retaining walls, culverts, bridge abutments, cisterns, dams and other such structures.
Building these structures, however, often requires separate concrete pours, that is one pour for the first horizontal element of the structure followed a second pour for the vertical element of the structure and sometimes additional pours are needed just for continuing an extremely long horizontal surface. Waiting for one element of the structure to dry before starting the second pour results in an imperfect mating of the two adjoining elements of the structure since there is no adherence between dry and wet concrete. This imperfect mating plus the normal concrete shrinkage that occurs as concrete dries can create a passage for water.
To prevent this problem, a number of solutions have been developed. The most popular is the use of PVC strips known in the industry as PVC waterstops. These are long strips inserted vertically and halfway into fresh concrete and when the second pour is done, the PVC waterstop is totally immersed into concrete and will act as a dam for water that would normally follow the passage between the two pours.
PVC waterstops currently in use are far from perfect and one of the inconveniences of using them is that since polyvinyl chloride has zero adhesion with concrete, the smallest shrinkage of concrete, which is normal during the curing process. Even for walls 150 mm thick, it can take 850 days for moisture to drop to below 50% at the center, as is described in an information brochure published by Portland cement. As moisture level drops, shrinkage occurs which creates a gap between the concrete and the PVC waterstop since PVC doesn't adhere to concrete. This is when a passage for water is formed.
More and more contractors and consultants refuse to use or recommend the use PVC waterstops and do not want to be responsible for any leaks that should occur if PVC waterstops are used.
A newer method to seal concrete joints involvesd the use of a hydro expansive compound, the most popular being EPDM (Ethylene Propylene Diene Monomer) combined with an hydro expansive resin, but other such compounds can offer similar properties. The hydro expansive compound is cut into long strips that are slightly narrower than the width the second pour will be and is laid flat on top of the first pour, after it has dried and just before the second pour. After both pours have cured and shrinkage has created a passage for water, the hydro expansive compound inflates as it gets in contact with water. By inflating, it is able to block the passage of water.
The use of the hydro expansive compound in this fashion is not without flaws however. The curing process of concrete is quite complex and must be understood in order to realize why this approach is flawed:
Due to segregation and bleeding, the uppermost layer of cured concrete is more fragile and brittle, this layer is about 0-5 mm in thickness and is characterized by a white powder on the surface. It is necessary to remove this fine layer by using various abrading means such as sandblasting or high pressure water. This has to be done before laying the hydro expansive compound. This can fix half of the problem but this bleeding and segregation can also occur at the bottom of the second pour for which there is no way it can be fixed. Moreover, another factor to consider in making separate pours is that if the first pour is unusually dry, it will absorb moisture from the second pour and upset the water to concrete ratio and if the first pour is too humid, again it can upset the ratio of the second pour. This also affects a layer about 0-5 mm in thickness at the junction between the two pours where the concrete can be more fragile. Also, in the case of a vertical structure, such as a wall, the higher the wall is, the harder it is to get a good compacting of the concrete by way of a vibrator. This zone of higher risk of porosity is situated at between 0-20 mm in height starting from the joint between the two pours.
Since the hydro expansive compound lays flat, it cannot handle the problem of difficult compacting in the 0-20 mm zone and although the hydro expansive compound can stop water at the joint, another passage for water can be created just above it, rendering the hydro expansive compound less efficient.
Because both the PVC waterstops and the hydro expansive compound are deficient, there is a need for a better waterstop.